1. Technical Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer, and a communication apparatus using the same.
2. Related Art of the Invention
There has been known a dielectric filter used for a mobile communication unit, which has a single dielectric block in which a plurality of resonator holes are provided. The resonator holes have large-diameter hole sections and small-diameter hole sections mechanically connected to each other, and also have step portions between the large-diameter hole sections and the small-diameter hole sections. Inner conductors are formed on the inner surfaces of the resonator holes, and an outer conductor is formed on almost all outer surfaces of the block except for one opening end face among both end faces where the resonator holes have openings. The inner conductors are electrically disconnected (open) at one opening end face (open-circuited end face) and electrically short-circuited (connected) at the other opening end face (short-circuited end face). A pair of input and output electrodes are formed on outer surfaces of the dielectric block without being connected to the outer conductor.
The center frequency of such a dielectric filter depends on the conductive-path length of the inner conductors from the open-circuited end face to the short-circuited end face. The length of the inner conductors is set to .lambda./4, where .lambda. indicates the wavelength at the center frequency. The center frequency of the dielectric filter becomes lower as the conductive-path length increases, and the center frequency becomes higher as the conductive-path decreases. Therefore, to make the dielectric filter compact by reducing the size in the axial direction (the direction from the open-circuited end face to the short-circuited end face) of the resonator holes without changing the center frequency, it is necessary to increase the ratio of the diameter of the large-diameter hole sections to that of the small-diameter hole sections to make the conductive-path length of the inner conductors equal to that in the filter before being made compact.
Since the distance between the axes of adjacent resonator holes, which specifies the degree of coupling in the dielectric filter, is set to a certain length, however, the diameter of the large-diameter hole sections is limited. On the other hand, it is difficult to extremely reduce the diameter of the small-diameter hole sections in terms of the forming technologies of the dielectric filter.
In addition, since the conventional dielectric filter has a low impedance when viewed from the input and output electrodes, the filter needs to be connected to an external circuit through a capacitor or other elements. Therefore, a space required for installing a capacitor needs to be held and the capacitor needs to be soldered to the dielectric filter, which is complicated.